1. Field of the Invention
The present invention generally relates to a two-cycle combustion engine of an air scavenging type suitable for use as a power plant for a compact rotary machine such as, for example, a brush cutter and, more particularly, to the two-cycle combustion engine of a kind wherein a two-staged piston is drivingly inserted in a correspondingly two-staged cylinder bore.
2. Description of the Related Art
The two-cycle combustion engine is generally of a design in which an air-fuel mixture introduced into a crank chamber is supplied into a combustion chamber to scavenge a combustion gas within the combustion chamber. Accordingly, the conventional two-cycle combustion engine is susceptible to a problem in that a portion of the air-fuel mixture supplied into the combustion chamber tends to flow outwardly through an exhaust port together with the combustion gas, that is, susceptible to a so-called blow-off phenomenon. In view of this, in order to avoid the blow-off phenomenon, attempts have been made to provide a two-cycle combustion engine in which a two-staged piston having small and large diameter piston portions is drivingly accommodated within a cylinder bore having small and large diameter bore portions.
More specifically, as the two-cycle combustion engine of the type discussed above, the Japanese Laid-open Patent Publication No. 5-118225 discloses the two-cycle combustion engine so designed and so configured that the air-fuel mixture introduced into a pump chamber defined between the large diameter bore portion of the two-staged cylinder bore in the cylinder block and the small diameter piston portion of the two-staged piston is fed under pressure into the combustion chamber from the pump chamber through a connecting passage during the ascending motion of the two-staged piston and that an intake valve for selectively opening or closing an intake port of the combustion chamber is opened at a timing at which an exhaust port is substantially closed by the small diameter piston portion of the two-staged piston then ascending to thereby prevent the air-fuel mixture, then introduced into the combustion chamber through the intake port, from flowing outwardly through the exhaust port.
Also, the Japanese Examined Patent Publication No. 57-45890 discloses the two-cycle combustion engine in which an injection nozzle is employed for injecting an air into the exhaust passage and an air is introduced into an auxiliary chamber defined between the large diameter bore portion of the two-staged cylinder bore and the small diameter piston portion of the two-staged piston, by opening a check valve during a descending motion of the two-staged piston and in which in synchronism with the last stage of the exhaust stroke during which the exhaust port is gradually closed as a result of the subsequent ascending motion of the two-staged piston, the air pressurized within the auxiliary chamber can be blown into the exhaust passage through the injection nozzle to thereby urge the air-fuel mixture, once discharged into the exhaust passage through the exhaust port, backwardly into the cylinder bore.
It has, however, been found that the first mentioned two-cycle combustion engine disclosed in the Japanese Laid-open Patent Publication No. 5-118225 has a problem in that it requires a mechanism for driving the intake valve for selectively opening or closing the intake port, through which the air-fuel mixture can be introduced into the combustion chamber, by means of a cam shaft or a crankshaft. In addition, the first mentioned two-cycle combustion engine requires a carburetor for supplying the air-fuel mixture into the pump chamber, an air cleaner for supplying an air into the crank chamber and the intake valve for selectively opening or closing the intake port through which the air-fuel mixture can be supplied from the pump chamber into the combustion chamber to be disposed having been spaced a distance from each other, resulting in complication in structure and increase in cost. Yet, since the air-fuel mixture is introduced into the combustion chamber during opening of the intake valve prior to the exhaust port being completely closed, it is not possible to completely eliminate the blow-off phenomenon of the air-fuel mixture.
On the other hand, the second mentioned two-cycle combustion engine disclosed in the Japanese Examined Patent Publication No. 57-45890 has a problem in that since the air-fuel mixture once discharged into the exhaust passage through the exhaust port is forced to return into the cylinder bore by the action of the air injected through the injection nozzle, the efficiency with which the blow-off phenomenon can be effectively prevented is low and, also, since a large amount of air and a high air pressure are necessary, the auxiliary chamber tends to increase in size. Also, since the timing at which the air under pressure is injected must be set to the last stage of the exhaust stroke, the exhaust port tends to be closed by the two-staged piston then ascending before the air-fuel mixture once entering the exhaust passage is urged backwardly into the cylinder bore and, therefore, the air-fuel mixture is no longer returned into the combustion chamber. For these reasons, even in this two-cycle combustion engine, the blow-off phenomenon of the air-fuel mixture cannot be avoided effectively.